1. Field of the Invention
The invention relates to an epidural needle for administering a liquid anesthesia into the epidural space of a spine.
2. Description of the Related Art
There are two optional procedures for administering a liquid anesthesia into the epidural space of a spine. In the first procedure, an epidural needle is inserted into the epidural space, and the anesthesia is administered directly through the epidural needle. The second procedure employs an epidural needle to introduce a catheter into the epidural space. The epidural needle then is withdrawn, and the catheter is left in place. The catheter then may be used to administer anesthesia over a longer period of time.
Care must be taken with both optional approaches to avoid penetrating the dura matter, which is a delicate membrane over the arachnoid. A puncture of the dura matter can cause a leakage of spinal fluid and various post-operative problems, such as severe headaches.
U.S. Pat. No. 4,721,506 shows an epidural needle for the direct administration of a liquid anesthesia into the epidural space. This prior art needle cannula is symmetrically generated about a linear longitudinal axis, and includes a generally planar bevel at the distal end. The bevel intersects the longitudinal axis of the needle at an angle of approximately 45.degree.. The extreme tip defined by the bevel then is blunted at an angle of 80.degree.-100.degree. to the longitudinal axis and is rounded slightly from side-to-side. The blunting and rounding of the distal tip is intended to reduce the risk of inadvertent damage to the dura mater.
The prior art epidural needle for introducing a catheter into the epidural space differs from the configuration shown in U.S. Pat. No. 4,721,506 in several respects. First, a needle for introducing a catheter typically must be cross-sectionally larger than the epidural needle for the direct administration of a liquid anesthesia. For example, an epidural needle for introducing a catheter typically will have a gauge size of about 17-18 gauge (iso-9626). Additionally, a catheter that is urged axially beyond the end of a linear needle conceivably could damage the dura matter. Consequently, the prior art epidural needle for introducing a catheter typically will be configured to direct the catheter at an angle to the axis of the needle. The requirement for directing the catheter at an angle further complicates the various design requirements of the needle. First, the distal tip of the needle must be sufficiently sharp to penetrate of outer layers of the skin and then to enable penetration of the ligamentum flavum without significant trauma to the tissue. Second, the distal end must be curved to direct the catheter into an off-axis alignment. Third, the distal tip must be sufficiently blunted to minimize the potential for inadvertent contact-related damage to the dura mater.
A prior art attempt for accommodating these different competing objectives is shown in U.S. Pat. No. 5,843,048 which is directed to a needle for introducing a catheter into the epidural space. A side wall of the epidural needle shown in U.S. Pat. No. 5,843,048 is curved near the distal tip of the needle. A bevel then is formed to intersect the curved side wall at the distal tip. Finally, the extreme distal end where the beveled surface and the curved side wall intersect is blunted to define a secondary bevel surface. The secondary bevel shown in U.S. Pat. No. 5,843,048 defines a substantially planar surface that is aligned to the longitudinal axis of the needle at an angle of approximately 80.degree.-100.degree.. Thus, the plane defined by the secondary bevel lies within a 10.degree. variation from a plane passing perpendicularly through the longitudinal axis. This is substantially the same angular alignment of blunting employed on the straight epidural needle disclosed in U.S. Pat. No. 4,721,506.
The blunting or secondary bevel on the prior art epidural needles both are intended to minimize the risk of damage caused by inadvertent contact with the dura matter. However, the prior art blunt tip also necessitates a substantially higher force for the initial penetration of the skin and for puncturing the ligamentum flavum. The requirement for higher forces with the prior art blunted epidural needle necessarily implies greater trauma to the patient. Furthermore, the greater forces that must be exerted on the prior art epidural needle creates the potential that the prior art epidural needle will accelerate after penetrating the ligamentum flavum, and hence will be propelled into the dura matter. Thus, the blunted tip can actually lead to the dural puncture that the prior art blunted tip is intended to avoid.
In view of the above, it is an object of the subject invention to provide an epidural needle with a distal end configured to both facilitate entry into the epidural space and reduce risk of damage to the dura mater.